The present invention relates to a power transistor module for use in a power switching device, and to its internal wiring structure.
Products with power transistor modules have become widely available, including a bridge circuit with IGBT's (Insulated Gate Bipolar Transistors) as power switching elements, for example. In such modules, several interconnected transistor chips are assembled in the same package.
A prior art module is illustrated by FIGS. 10a and 10b for a half bridge structure with two power transistors. FIG. 10a shows a metal base 1 for heat dissipation, an insulating substrate 2 for the circuit, a copper foil circuit pattern 3 formed on the insulating substrate, a power transistor chip 4 such as an IGBT chip (herein referred to as "transistor" for short), a freewheel diode chip 5 (herein referred to as "freewheel diode" or simply "diode" for short) connected in parallel with the transistor 4, external output terminals 6, 7 and 8 for use with a main circuit, signal terminals 9 and 10 taken out from auxiliary emitter terminals for each of the transistors, an internal lead 11 connecting together the signal terminals 9 and 10 and the external output terminals 7 and 8, and bonding wires 12 connecting the transistors 4 and diodes 5 to the circuit pattern 3. A circuit substrate such as a DBC substrate (Direct Bonding Copper substrate) or an insulated aluminum substrate may be used as the circuit substrate 2. Within parentheses, FIG. 10a includes symbols in correspondence with elements and terminals of the equivalent circuit of FIG. 10b. Specifically designated are power transistors Tr1 and Tr2 for the upper and lower arms of the bridge circuit, freewheel diodes D1 and D2, collector terminal C1 for the transistor Tr1, the common terminal C2E1 for the emitter of the transistor Tr1 and the collector of the transistor Tr2, the emitter terminal E2 for the transistor Tr2, signal terminals (auxiliary emitter terminals) e1 and e2 for the transistors Tr1 and Tr2, and gate terminals G1 and G2.
As shown in FIG. 10, the external output terminal 6 is taken out from the collector pattern part 3a on which the transistor Tr1 and the diode D1 are mounted, and the external output terminal 7 is taken out from the collector pattern part 3b on which the transistor Tr2 and the diode D2 are mounted. A bonding wire 12 connects the collector pattern part 3b to the emitter electrode of the transistor Tr1 and the diode D1. The external output terminal 8 (E2) is taken out from the emitter pattern part 3c for the transistor Tr2. The signal terminals 9 (e1) and 10 (e2) as the auxiliary emitter terminals are connected to the terminal pads for the respective external output terminals 7 (C2E1) and 8 (E2) with leads 11 at positions which provide internal wiring inductances L1 and L2 shown in the equivalent circuit of FIG. 10b. The gate terminals G1 and G2 are taken out from a gate pattern part connected by wire to the gate electrodes for the transistors Tr1 and Tr2.
The internal wiring inductances L1 and L2 are provided for inducing a voltage due to counter-electromotive force at turn-off of the transistors. The induced voltage is applied to the gate so that it weakens the drop of the gate voltage and reduces the magnitude of -di/dt, so that a gate-emitter voltage surge is suppressed. For balanced switching operation between the transistors Tr1 and Tr2, it is necessary for the wiring inductances L1 and L2 to have suitably chosen values.
In this device, difficulties arise at increased switching frequencies, due to the internal wiring inductance L1 provided by the signal terminal e1 for the driving signal of the transistor Tr1.
This will be described with reference to FIG. 11 which shows the load current flow in the circuit of FIG. 10b during switching. When transistor Tr1 is on and transistor Tr2 is off, the emitter current Ie of the transistor Tr1 flows to the load L via the inductance L1. On the other hand, when the transistor Tr1 is turned off and the collector current Ic becomes zero, a freewheeling current If flows continuously in the same direction as Ie via the diode D2 into the internal wiring inductance L1 as a result of the freewheeling operation of the diode D2. Then, no sufficient counter-electromotive force is induced in the internal wiring inductance L1. This does not sufficiently suppress the above-mentioned voltage surge, and results in malfunctioning in the transistor drive circuit and in externally connected equipment to be controlled.
Contrary to this, in the transistor Tr2 of the lower arm, the current which flows through the internal wiring inductance L2 on the side of the signal terminal e2 varies according to the turning on or turning off of the transistor Tr2, since the freewheeling current does not flow through the inductance L2). Therefore, the voltage due to the counter-electromotive force generated in the inductance L2 is applied to the drive circuit for the transistor Tr2 via the signal terminal e2, so that the voltage surge can be suppressed.
Thus, with prior art internal wiring structure for of power transistor module, large voltage surges are produced in high speed switching especially in the upper arm including the transistor Tr1. Also, a difference arises in the size of a voltage rise between the transistor Tr1 on the upper arm and the transistor Tr2 on the lower arm. This causes unstable switching characteristics of the module.